The present invention relates to electronic circuits, and more particularly to techniques relating to oscillators.
FIG. 1 illustrates a configuration of a prior art voltage-controlled oscillator (VCO) 104. VCO 104 is part of a phase-locked loop (PLL) circuit. The PLL includes a PLL loop filter 101, a voltage buffer 102, a low pass filter (LPF) 103, and VCO 104. PLL loop filter 101 generates a control voltage VCL that is based on the difference between the phase and the frequency of a reference clock signal and the phase and the frequency of a feedback clock signal. Voltage buffer 102 buffers VCL to generate an oscillator supply voltage VOS.
A supply voltage VCC is filtered by LPF 103 to generate a filtered supply voltage VFL. The filtered supply voltage VFL is provided to a supply input of voltage buffer 102. VCO 104 is a ring oscillator that includes two differential VCO cells 105-106. VCO cells 105-106 are differential inverting delay circuits. The oscillator supply voltage VOS is provided to supply inputs of VCO cells 105 and 106. VCO cell 106 generates a differential output signal OUT/OUTB.
FIG. 2 is a schematic diagram of a prior art VCO cell that is used to implement VCO 104. The VCO cell circuit structure shown in FIG. 2 is in VCO cell 105. The VCO cell circuit structure shown in FIG. 2 is also in VCO cell 106. VCO cell 105/106 includes p-channel metal oxide semiconductor field-effect transistors (MOSFETs) 201-202 and n-channel MOSFETs 203-206. INP is the non-inverting (+) input of VCO cell 105/106, and INN is the inverting (−) input of VCO cell 105/106. OUTP is the non-inverting output of VCO cell 105/106, and OUTN is the inverting output of VCO cell 105/106. The inverting outputs of VCO cells 105/106 are represented by circles in VCO 104. The oscillator supply voltage VOS is provided to the sources of transistors 201-202.
FIG. 3 is a graph that illustrates the frequency response and the gain of VCO 104. The frequency response of VCO 104 refers to the frequency of differential output signal OUT/OUTB. The gain (KVCO) of VCO 104 refers to the change in frequency of OUT/OUTB versus the change in the control voltage VCL. VCO 104 provides a tuning range for OUT/OUTB from about 1-14 GHz as shown in FIG. 3. The gain of VCO 104 varies from about 7 GHz/volt to a maximum of about 12 GHz/volt over the tuning range of VCO 104.
VCO 104 is very susceptible to generating jitter in output signals OUT and OUTB in response to supply voltage noise in VOS, particularly at high frequencies. Noise in VOS can, for example, be generated in response to noise in VFL or noise in VCL.